The flow cross section is changed in such prior-art air intake fittings by perforated disks arranged in the axial opening, with which the free flow cross section is varied by mutual rotation. In this embodiment, the closed areas of the perforated disks to be mutually rotated may lead to a small maximum flow cross-sectional area. Furthermore, dirt particles may be easily drawn into the heater with the combustion air drawn in in the case of axial flow of the combustion air into the air intake fitting.